Geologic Vulnerability - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 196

additional level of interpretation consisting of a comprehensive vulnerability analysis is
added to the near-surface geologic maps of urban areas presented in Chapter 5. This chap-
ter also explains why certain types of geology may be especially susceptible to contamina-
tion—a topic explored and discussed in greater detail later in this topic.
6.2 Subsurface Vulnerability and Vulnerability Map Development
The concept of vulnerability of the subsurface to contamination originated in France
during the 1960s and was introduced into the scientific literature by Albinet and Margat
(1970). Since then, the concept of subsurface vulnerability has evolved to include both a
distinction between and combination of vulnerability and risk assessment. Groundwater
vulnerability is currently interpreted as a function of the natural properties of the overly-
ing soil or sediments of the unsaturated zone, aquifer properties (e.g., effective porosity
and recharge area), and aquifer material (Foster and Hirata 1988; Robins et al. 1994; Rogers
et al. 2007).
Geologic vulnerability mapping can be divided into two groups: subjective rating
methods and statistical and process-based methods (Focazio et al. 2001). The subjective
rating methods are characterized by numerical scales representing low to high vulner-
abilities. Typically the results are applied to large areas and used for policy and manage-
ment objectives. By contrast, the statistical process-based methods produce finite values,
such as areas exceeding specific water quality values. With these methods, the results are
usually not applied to large areas due to data gaps and variable geology. In addition, the
results are generally obtained under more detailed site-specific assessments and used for
purely scientific purposes (Focazio et al. 2001). In practice, the subjective rating methods
are preferred for conducting vulnerability assessments on a watershed scale (Murray and
Rogers 1999a).
The concept of geologic vulnerability relies on the assessment and representation of
various hydrogeologic parameters such as vadose zone characteristics (e.g., thickness and
infiltration capacity), depth to water, and amount of recharge (Zaporozec and Eaton 1996;
Eaton and Zaporozec 1997). The utility of this concept, however, becomes more important
when the geologic data are supplemented with environmental, economical, and political
insight gained through past environmental cleanup efforts (Foster et al. 1993; Loague et al.
1998). A specific example of this data augmentation is provided at the end of the chapter.
Successful development of geologic vulnerability maps can be difficult to achieve in
areas experiencing rapid growth. As noted in Chapters 2 and 3, urbanization and the arti-
ficial infrastructure it produces (e.g., sewers and detention ponds) can have a profound
influence on the regional hydrogeology (Vuono and Hallenbeck 1995; Zaporozec and Eaton
1996; Kibel 1998). Basic processes affecting surface water and groundwater are modified,
including surface water drainage patterns and velocities, evaporation rates, infiltration,
and aquifer recharge (Burn et al. 2007; Garcia-Fresca 2007; Howard et al. 2007; Mohrlok
et al. 2007). The difficulties in vulnerability map development are also compounded by the
differences in the amount and type of geologic and hydrogeologic information available
in urban areas and rural settings. For these reasons, a uniform assessment of data while
conducting vulnerability studies in urbanizing areas is difficult to achieve.
Geologic vulnerability mapping provides a starting point for quantifying anticipated
environmental risk at a particular site and can also highlight locations where additional